Vinylidene fluoridepolymer having a fraction of non-transferred chains and its manufacturing process

ABSTRACT

The invention is a PVDF (polyvinylidene fluoride) homopolymer or copolymer, the comonomer being chosen from compounds containing a vinyl group capable of being opened by the action of free radicals in order to be polymerized and which contains, directly attached to this vinyl group, at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxy group such that: 
         it comprises a fraction of non-transferred chains of very high molar mass which are completely insoluble in the usual solvents for PVDF, such as DMF (dimethylformamide), DMSO (dimethyl sulphoxide) and NMP (N-methylpyrrolidone), the polymer fraction associated with these high-mass chains having a dynamic viscosity of greater than 50 kpoise at 230° C. and at a shear rate of 100 s −1 , the size of the spherulites is between 0.5 and 4 μm. The invention also relates to blends of this PVDF with an ABC triblock copolymer, the three blocks A, B and C being linked together in this order, each block being either a homopolymer or a copolymer obtained from two or more monomers, the A block being linked to the B block and the B block to the C block by means of a covalent bond or of an intermediate molecule linked to one of these blocks via a covalent bond and to the other block via another covalent bond, and such that: the A block is compatible with PVDF, the B block is incompatible with PVDF and is incompatible with the A block, the C block is incompatible with PVDF, the A block and the B block. The invention also relates to the parts made of the above composition. These parts may be sheets, films, tubes, rods, centrifugal pump components and containers. The invention also relates to a process for synthesizing this PVDF, in which:    a dispersion of VF 2  (vinylidene fluoride) and of one or more optional comonomers in water is made, possibly with the aid of a surfactant, the said dispersion being initially brought into contact with a water-soluble non-organic initiator capable of causing the polymerization of the monomers; then, part of the PVDF having been formed in the presence of the water-soluble non-organic initiator, the following are added: either (i) a chain transfer agent capable of propagating the polymerization, the said polymerization then being initiated by a water-soluble non-organic initiator or by an organic initiator, or (ii) an organic initiator, also capable of accomplishing chain transfer, and optionally a water-soluble non-organic initiator.

FIELD OF THE INVENTION

Polymers based on vinylidene fluoride (VF₂), such as PVDF(polyvinylidene fluoride) for example, are known to provide excellentmechanical stability properties, very high chemical inertness and goodageing resistance. These properties are exploited in varied fields ofapplication. Mention may be made, for example, of the manufacture ofextruded or injection-moulded parts for the chemical engineeringindustry or for microelectronics, use in the form of sealing sleeves forthe transportation of gases or hydrocarbons, production of films orcoatings providing protection in the architectural field and productionof protective components for electro-technical uses.

PRIOR ART AND TECHNICAL PROBLEM

VF₂-based fluoropolymers and particularly PVDF have an impact strengthwhich is not always sufficient. This drawback may have dramaticconsequences as it may lead to the fracture of a part, either after amechanical impact or after an unplanned period of time within atemperature range in which the toughness of the material is usually verypoor (T=−30° C.). The prior art has already described impact-resistantPVDF compositions.

Patent EP 884 358 describes flexible and tough poly(vinylidenefluoride)-based compositions. They comprise, per 100 parts by weight ofa vinylidene fluoride (VF₂) homopolymer (A) or a copolymer (A) of VF₂with at least one other monomer copolymerizable with VF₂, in whichcopolymer, per 100 parts by weight of VF₂, the said monomer is presentin an amount of between 0 and 30 parts by weight, from 0.5 to 10 partsby weight of an elastomer B and from 0.5 to 10 parts by weight of aplasticizer C, with the additional condition that the sum of B plus C befrom 1 to 10.5 parts by weight and, moreover, that the vinylidenefluoride homopolymer (A) be chosen in such a way that it has a melt flowindex, measured according to the ISO 1133 standard at 230° C. with aload of 5 kg, of less than 5 g/10 min and a critical modulus G_(C),measured at 190° C., at the intersection of the shear moduli in the meltG′ and G″, of between 5 and 22 kPa. In general, they are suitable forthe production of objects, articles such as sheets, films, sheaths ofpipes, hoses, etc., subjected to stresses under high and/or lowtemperature conditions, in contact with particularly aggressivesubstances (such as hydrocarbons, strong acids, solvents, mineral andorganic bases) during which their toughness and flexibility propertiesare particularly required (oil and gas industries, chemical engineering,construction industries and civil engineering).

International Application WO 99/29772 describes the reinforcement ofPVDF with a poly(styrene)-poly(butadiene)-poly(methyl methacrylate)triblock copolymer. The PVDF thus modified retains its chemicalresistance properties.

The incorporation of a plasticizing additive or elastomer is effective,but it poses problems of homogenization of the blend and of thermal orchemical stability depending on the nature of these additives. Inaddition, producing a formulation with plasticizers and/or elastomersrepresents an additional step in the manufacture of the final object.

It has now been found that a PVDF having a fraction of non-transferredchains has very good impact strength without it being necessary to addplasticizers or impact modifiers thereto. This polymer will be termed inthe rest of the text “PVDF having a fraction of non-transferred chains”(FNTC). That is to say that this PVDF comprises very high-mass chains,namely non-transferred chains, and other PVDF chains which were producedby initiation and transfer. A very simple means of manufacturing thisPVDF having a FNTC consists in starting the polymerization of VF₂ withan initiator which does not induce a transfer reaction, and in theabsence of a transfer agent, then, after part of the PVDF has beenformed, that having a very high molar mass, the transfer agent (alsocalled chain regulator) is added. The chains of very high molar masswhich correspond to the FNTC are completely insoluble in the usualsolvents for PVDF, such as DMF (dimethylformamide), DMSO (dimethylsulphoxide) and NMP (N-methylpyrrolidone). In addition, the polymerfraction associated with the FNTC has a dynamic viscosity of greaterthan 50 kpoise at 230 C and at a shear rate of 100 s⁻¹.

The prior art has described, in U.S. Pat. No. 4,076,929, a PVDF called abimodal PVDF, but it is very different from that of the presentinvention. In this prior art, the polymerization of VF₂ is carried outin the presence of a surfactant and di-tert-butyl peroxide as initiatorwithout adding a chain transfer agent. In fact, the tert-butyl peroxidealso acts as a chain transfer agent since some of the hydrogens of itshydrocarbon part are labile. This transfer function is much weaker thana specific transfer agent such as ethyl acetate, but it is sufficientfor the PVDF chains not to achieve very high masses, as in the presentinvention. It is therefore essential to produce high-mass chains—it isspecified in column 2 lines 45-52 that the proportion of high-masschains is from 30 to 70% and preferably at least 45%.

The prior art has described many processes for preparing PVDF, but aprocess has never been described in which the transfer agent is addedafter the start of a polymerization initiated specifically by awater-soluble non-organic initiator which does not induce a transferreaction. As a reminder, mention may be made of the following patents:

Patent EP 387 938 describes the polymerization of VF₂ in the absence ofan emulsifier with the aid of a peroxydisulphate as initiator and of achain regulator added to the initial monomer charge. Patent EP 655 468describes the polymerization of VF₂ in the presence of a radicalinitiator and of a chlorofluoroalkane (HCFC 123) as transfer agent. HCFC123 is added either altogether at the start of the polymerization or infractions during the polymerization, and it is very clear according toExample 1 that a fraction is added before the polymerization. Patent EP169 328 describes the polymerization of VF₂ in the presence of asurfactant, an initiator, trichlorofluoromethane and isopropyl alcohol,the last two being transfer agents. The initiator is always added aftersome or all of the transfer agents. Patent FR 2 259 114 describes thepolymerization of VF₂ in the presence of a radial initiator and a chaintransfer agent, in which the initiator and the transfer agent aregradually added to the polymerization reactor at the same time as theVF₂ is added to the reactor in small proportions.

Another technical problem has also been solved. PVDF sometimes has toocoarse a crystalline morphology, that is to say one consisting ofcrystalline entities (called spherulites) of too large a mean size or oftoo broad a size distribution. Among the negative consequences of such amorphology may be distinguished, for example, a high microroughness ofobjects obtained by melt processing, lack of gloss or a poor surfacefinish, and a lower transparency of the final material. In addition, adegradation in the mechanical properties upon prolonged contact withcorrosive chemical substances may also result from a surface crystallinemorphology which is not fine enough. The prior art has described meansfor reducing the mean size of the spherulites; nucleating agents aregenerally introduced into the fluoride-based matrix.

Japanese Patent Application JP48-34956 discloses a process for preparinga PVDF-based compound, consisting in mixing, into the PVDF latex orparticles, from 0.05% to 30% of a fluororesin latex whose melting pointis greater than that of PVDF. Such a fluororesin may be poly(vinylfluoride), polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene,or else resins chosen from homopolymers and copolymers of vinyl fluoride(VF), chlorotrifluoroethylene (CTFE), vinylidene fluoride (VF₂) anddifluorodichloroethylene (VF₂Cl₂), resins whose melting point is higherthan that of PVDF. From 0.05 to 30% by weight with respect to the latexPVDF of these fluororesins are normally added, the resin particleshaving a diameter of 0.05 to 1 μm. In Example 8, PVDF blended with 0.05μm PTFE particles gave, after melting the PVDF at 250° C., a 1-mm thickplaque comprising spherulites of less than 1 μm in size.

Patents DE 2 116 847 and FR 2 721 036 describe the addition of aromaticor heterocyclic molecular compounds.

The effectiveness of certain nucleating agents has been clearlydemonstrated, but their incorporation into a polyvinylidene fluoridematrix is always a tricky step. This is because the concentrationsrequired are very low, typically of the order of 0.1% by weight, whichmakes it difficult to obtain a homogeneous dispersion.

A PVDF has now been discovered which has mean spherulite sizes of theorder of 1 μm—this is the abovementioned PVDF having a fraction ofnon-transferred chains of the invention. It is manufactured without theaddition of nucleating agents.

BRIEF DESCRIPTION OF THE INVENTION

The invention is a PVDF (polyvinylidene fluoride) homopolymer orcopolymer, the comonomer being chosen from compounds containing a vinylgroup capable of being opened by the action of free radicals in order tobe polymerized and which contains, directly attached to this vinylgroup, at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxygroup such that:

-   -   it comprises a fraction of non-transferred chains of very high        molar mass which are completely insoluble in the usual solvents        for PVDF, such as DMF (dimethylformamide), DMSO (dimethyl        sulphoxide) and NMP (N-methylpyrrolidone), the polymer fraction        associated with these high-mass chains having a dynamic        viscosity of greater than 50 kpoise at 230° C. and at a shear        rate of 100 s⁻¹,    -   the size of the spherulites is between 0.5 and 4 μm.

The invention also relates to blends of this PVDF with an ABC triblockcopolymer, the three blocks A, B and C being linked together in thisorder, each block being either a homopolymer or a copolymer obtainedfrom two or more monomers, the A block being linked to the B block andthe B block to the C block by means of a covalent bond or of anintermediate molecule linked to one of these blocks via a covalent bondand to the other block via another covalent bond, and such that:

-   -   the A block is compatible with PVDF,    -   the B block is incompatible with PVDF and is incompatible with        the A block,    -   the C block is incompatible with PVDF, the A block and the B        block.

The invention also relates to the parts made from the above composition.These parts may be sheets, films, tubes, rods, centrifugal pumpcomponents and containers.

The invention also relates to a process for synthesizing this PVDF, inwhich:

-   -   a dispersion of VF₂ (vinylidene fluoride) and of one or more        optional comonomers in water is made, possibly with the aid of a        surfactant, the said dispersion being initially brought into        contact with a water-soluble non-organic initiator capable of        causing the polymerization of the monomers;    -   then, part of the PVDF having been formed in the presence of the        water-soluble non-organic initiator, the following are added:        -   either (i) a chain transfer agent capable of propagating the            polymerization, the said polymerization then being initiated            by a water-soluble non-organic initiator or by an organic            initiator,        -   or (ii) an organic initiator, also capable of accomplishing            chain transfer, and optionally a water-soluble non-organic            initiator.

The principle of this process is based on the formation, at the start ofpolymerization, of a fraction of macromolecular chains of very highmolar mass, produced without a transfer agent (or without a sidereaction of the transfer or termination type contributing to the chainlength being greatly limited) and without an initiator capable ofinducing a transfer reaction. The reaction therefore starts without atransfer agent (CTA) and the first charge of CTA is injected at a degreeof conversion of the monomers of, for example, around 5% by weight. Thenecessary dose of CTA can then be introduced in increments orcontinuously, the total amount allowing the average molar mass of thepolymer to be adjusted. In the case of a single injection of transferagent, the product obtained will exhibit a bimodal distribution of themolar masses with a first population of very high mass and a secondpopulation of limited mass. The polymerization step after the first doseof transfer agent has been added may also be carried out under theeffect of an organic initiator whose contribution to the transferreactions will be of a greater or lesser importance.

In the particular case of an organic initiator having a transfer effectsufficient to adjust the molar mass, it is also possible to dispensewith the actual transfer agent without changing the nature of theinvention. In this case, the FNTC is still obtained during the firststep of the polymerization in the presence of the non-organic initiator,and a second fraction of moderate molar mass is formed under the soleaction of the organic initiator.

The invention makes it possible to improve the impact strengthproperties and the fineness of the morphology of PVDF merely bymodifying the synthesis conditions, and more specifically by optimizingthe way in which the transfer agent is introduced during polymerizationand by initially using a water-soluble non-organic initiator. Thisprocess change takes place without any change to the apparatus formanufacturing the polymer and can thus be very rapidly transposed to anindustrial production line.

DETAILED DESCRIPTION OF THE INVENTION

As examples of comonomers, mention may be made of vinyl fluoride;trifluoroethylene; chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene;tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkylvinyl) ethers, such as perfluoro(methyl vinyl) ether (PMVE),perfluoro(ethyl vinyl) ether (PEVE) and perfluoro(propyl vinyl) ether(PPVE); perfluoro(1,3-dioxole); perfluoro(2,2-dimethyl-1,3-dioxole)(PDD); the product of formula CF₂═CFOCF₂CF(CF₃)OCF₂CF₂X in which X isSO₂F, CO₂H, CH₂OH, CH₂OCN or CH₂OPO₃H; the product of formulaCF₂═CFOCF₂CF₂SO₂F; the product of formula F(CF₂)_(n)CH₂OCF═CF₂ in whichn is 1, 2, 3, 4 or 5; the product of formula R₁CH₂OCF═CF₂ in which R₁ ishydrogen or F(CF₂)_(z) and z is 1, 2, 3 or 4; the product of formulaR₃OCF═CH₂ in which R₃ is F(CF₂)_(z)— and z is 1, 2, 3 or 4;perfluorobutylethylene (PFBE), 3,3,3-trifluoropropene and2-trifluoromethyl-3,3,3-trifluoro-1 -propene. Several comonomers may beused.

Advantageously, the PVDF is chosen from vinylidene fluoride (VF₂)homopolymers and copolymers preferably containing at least 50% by weightof VF₂, the comonomer being chosen from chlorotrifluoroethylene (CTFE),hexafluoropropylene (HFP), trifluoroethylene (VF₃) andtetrafluoroethylene (TFE).

With regard to the proportion of non-transferred chains (of very highmass), this may represent up to 50% by weight of the PVDF and isadvantageously between 2 and 30% by weight. This is the proportion withrespect to the total amount of PVDF, that is to say that for 2 to 30% ofchains of very high mass there are 98 to 70% of other chains,respectively. The proportion is preferably between 5 and 30% and betterstill between 15 and 25%. The insolubility of the chains of very highmass in the usual solvents for PVDF is found according to the rules ofthe art.

With regard to the size of the spherulites, this is advantageouslybetween 0.8 and 2 μm. Typically, for a standard PVDF prepared at 80° C.by an emulsion process in the presence of a surfactant, an initiator anda transfer agent, the mean size is between 2 and 10 microns. This sizeis found in Kynar® 740 and Kynar® 1000, which are also PVDFhomopolymers, whereas for a PVDF according to the present inventioncontaining 25% by weight of chains of very high mass the mean size is ofthe order of 1 μm. Kynar® 740 and Kynar® 1000 are described in thecomparative examples of the present invention.

Advantageously, the MFI (Melt Flow Index) is between 1 and 50 (in g/10min at 230° C.-/5 kg). Preferably, the MFI of the PVDF of the inventionis either between 1 and 3, and preferably between 1.5 and 2.5, orbetween 10 and 50, and preferably between 15 and 40.

The PVDFs of the invention exhibit good impact strength. In addition,they are highly pseudoplastic in nature (having a large drop inviscosity with shear rate), this generally being an advantage from themelt processing standpoint, and a newtonian plateau back towards the lowshear rates, this being useful, for example, for the extrusion-blowingof thin films.

With regard to the ABC triblock copolymer: The C blockopolymercomprising at least three blocks A, B and C is such that the A block islinked to the B block and the B block to the C block by means of one ormore single covalent bonds. In the case of several covalent bonds,between the A block and the B block and/or between the B block and the Cblock, there may be a single unit or a linked sequence of units servingto join the blocks together. In the case of a single unit, the lattermay come from a monomer, called a moderator, used in the synthesis ofthe triblock. In the case of a linked sequence of units, this may be anoligomer resulting from the linking of monomer units of at least twodifferent monomers in an alternating or random order. Such an oligomermay link the A block to the B block, and the same oligomer or adifferent oligomer may link the B block to the C block.

The A block of an ABC copolymer is regarded as being compatible withPVDF if the A polymer identical to this block (and therefore without Band C sequences) is compatible with this resin in the melt. Likewise,the A and B blocks are regarded as being incompatible if the A and Bpolymers identical to these blocks are incompatible. In general,compatibility between two polymers should be understood to mean theability of one to dissolve in the other in the melt, or else theircomplete miscibility. If this is not the case, the polymers or blocksare called incompatible.

The lower the enthalpy of mixing of two polymers, the greater theircompatibility. In some cases, there is a favourable specific interactionbetween the monomers which results in a negative enthalpy of mixing forthe corresponding polymers. In the context of the present invention, itis preferred to use compatible polymers whose enthalpy of mixing isnegative or zero.

However, the enthalpy of mixing cannot be conventionally measured forall polymers, and therefore the compatibility can only be determinedindirectly, for example by viscoelastic analytical measurements intorsion or in oscillation, or else by differential calorimetry. Forcompatible polymers, two glass transition temperatures or T_(g)s can bedetected for the blend: at least one of the two T_(g)s is different fromthe T_(g)s of the pure compounds and lies within the temperature rangebetween the two T_(g)s of the pure compounds. A blend of two completelymiscible polymers has a single T_(g).

Other experimental methods may be used to demonstrate the compatibilityof the polymers, such as turbidity measurements, light-scatteringmeasurements and infrared measurements (L. A. Utracki, Polymer Alloysand Blends, pp. 64-117).

Miscible or compatible polymers are listed in the literature—see, forexample J. Brandrup and E. H. Immergut : Polymer Handbook, 3rd Edition,Wiley & Sons 1979, New York 1989, pp. VI/348 to VI/364; 0. Olabisi, L.M. Robeson and M. T. Shaw : Polymer Miscibility, Academic Press, NewYork 1979, pp. 215-276; L. A. Utracki : Polymer Alloys and Blends,Hanser Verlag, Munich 1989. The lists appearing in these references aregiven by way of illustration and are not, of course, exhaustive.

Advantageously, the A block is chosen from homopolymers and copolymersof alkyl (alkyl)acrylates, for example methyl methacrylate (MMA) and/ormethyl or ethyl acrylate and/or those deriving from vinyl acetate.

Advantageously, the A block is poly(methyl methacrylate) (PMMA).

Preferably, this PMMA is syndiotactic and its glass transitiontemperature T_(g)(A), measured by differential thermal analysis, is from+120° C. to +140° C.

Advantageously, the T_(g) of B is less than 0° C. and preferably lessthan −40° C.

The monomer used to synthesize the elastomeric B block may be a dienechosen from butadiene, isoprene, 2,3-dimethyl-1,3-butadiene,1,3-pentadiene and 2-phenyl-1,3-butadiene. Advantageously, B is chosenfrom polydienes, especially polybutadiene, polyisoprene and their randomcopolymers, or else from partially or completely hydrogenatedpolydienes. Among polybutadienes, it is advantageous to use those whoseT_(g) is the lowest, for example poly(1,4-butadiene) whose T_(g) (around−90° C.) is less than that (around 0° C.) of poly(1,2-butadiene). Theblocks B may also be hydrogenated. This hydrogenation is carried outusing the standard techniques.

The monomer used to synthesize the elastomeric B block may also be analkyl (meth)acrylate; the following T_(g) values in brackets followingthe name of the acrylate are obtained: ethyl acrylate (−24° C.), butylacrylate (−54° C.), 2-ethylhexyl acrylate (−85° C.), hydroxyethylacrylate (−15° C.) and 2-ethylhexyl methacrylate (−10° C.). It isadvantageous to use butyl acrylate. The acrylates are different fromthose of the A block in order to meet the condition of B and A beingincompatible.

Preferably, the B blocks consist predominantly of poly(1,4-butadiene)Preferably, the C block has a glass transition temperature T_(g(C)) or amelting point T_(m(C)) greater than the T_(g(B)) of the B block. Thischaracteristic means that the C block can either be in the glassy stateor in a partially crystalline state and the B block can be in theelastomeric state, for the same service temperature T_(S).

According to the present invention, it is possible to choose the natureof the B blocks in order to have a certain defined T_(g(B)) and thus, atthe service temperature T_(S) of the material or of the article formedfrom the blend, to have these B polymer blocks in an elastomeric orflexible state. On the other hand, since the C polymer blocks can have aT_(g(C)) or a T_(m) greater than T_(g(B)), they may be in a relativelyrigid glassy state at the same service temperature.

Since the C blocks are incompatible with PVDF, the A blocks and the Bblocks, they form a rigid discrete phase within the material, formingnanodomains included in the material and serving as anchoring in theregion of one of the ends of each B block. The other end of each B blockis linked to an A block which has a strong affinity with PVDF. Thisstrong affinity provides a second anchoring in the region of the secondend of the B block.

Advantageously, the C block is chosen from styrene or α-methylstyrenehomopolymers or copolymers.

The triblocks which contain sequences deriving from alkyl(alkyl)acrylates may especially be prepared by anionic polymerization,for example using the processes described in Patent Applications EP 524054 and EP 749 987.

Preferably, the ABC triblock is poly(methylmethacrylate-b-butadiene-b-styrene).

The ABC triblock copolymer may contain, as by-products of its synthesis,a BC diblock copolymer and possibly the homopolymer C. The ABC triblockcopolymer may also contain, as by-products of its synthesis, an ABdiblock copolymer and possibly the homopolymer A.

This is because the synthesis of a triblock copolymer is preferablycarried out by joining, in succession, the A block to the B block andthen to the C block, or conversely the C block to the B block and thento the A block, depending on the nature of the three blocks A, B and C,the A block being by definition that block which is compatible withPVDF. The ABC triblock copolymer may also contain star or symmetricallinear block copolymers of the ABC or CBC type.

Advantageously, the total amount by weight of the synthesis by-products,that is to say these homopolymers A, C or AB, BC, ABA and CBC blockcopolymers, is less than twice the amount of triblock ABC. Preferably,this amount is less than one times and better still 0.5 times the amountof triblock ABC. More specifically, the by-products are essentially thediblock BC, it being possible for the amount of BC to be between 25 and35 parts by weight, per 75 to 65 parts of ABC respectively, andadvantageously about 30 parts per 70 parts of ABC.

The number-average molecular mass (Mn) of the triblock copolymer,including the synthesis by-products, is greater than or equal to 20 000g/mol, and preferably between 50 000 and 200 000 g/mol. Advantageously,the ABC triblock copolymer, including the by-products, consists of:

-   -   20 to 93 and preferably 30 to 70 parts by weight of A blocks;    -   5 to 68 and preferably 10 to 40 parts by weight of B blocks;    -   2 to 65 and preferably 5 to 40 parts by weight of C blocks.

The proportion of triblock ABC in the PVDF may be up to 30% by weightper 70% of PVDF. Advantageously, the triblock proportions are from 2 to30% per 98 to 70% of PVDF respectively, and preferably 5 to 15% per 95to 85% of PVDF respectively.

It would not be outside the scope of the invention to add to thesePVDFs, whether or not containing ABC triblocks, stabilizers, fireretardants, plasticizers and impact modifiers.

With regard to the process for synthesizing this PVDF: one fundamentalaspect of this process is that it relies on the use of (radical)initiator systems without any appreciable contribution to the transfer(or termination) reactions, either by their intrinsic chemical structureor by the chemical structure of their decomposition products. This isbecause, during the period of reaction without a CTA, minimumintervention of the transfer or termination reactions must be ensured.The present invention therefore applies particularly well to emulsionpolymerization processes as these make it possible to carry out theinitiation with water-soluble non-organic compounds such as, forexample, persulphates or hydrogen peroxide, which have no secondarytransfer effect. In contrast, most organic initiators that can be usedfor emulsion and/or suspension processes are not suitable because theyintervene in the transfer or termination reactions (owing to the labilehydrogens present in the hydrocarbon part of their backbone). Asexamples of these initiators having labile hydrogens in theirhydrocarbon part, mention may be made of di-tert-butyl peroxide. Ofcourse, it should be understood that it is only the initial chainfraction of very high molar mass which must be produced under theinfluence of an initiator without contribution to the transfer (ortermination) reactions. For the subsequent fractions formed in thepresence of a transfer agent, the same initiator or an organic-typeinitiator may be employed without restriction. The CTA is added when thechains of very high mass have been formed. It would not be outside thescope of the invention to use an initiator which also contributes to thetransfer such as, for example, an organic-type initiator. It would notbe outside the scope of the invention subsequently to add another amountof initiator in one or more stages, the same initiator or an initiatorcontributing to the transfer, possibly another amount of monomers in oneor more stages, and any combination of these possibilities.

The degree of conversion of the VF₂ and of the optional comonomersbefore the first injection of CTA determines the chain fraction formedwithout a transfer agent. The chains of very high mass having beenformed, the number of injections or the rate of introduction of the CTAthen determines the molar mass distribution of the fraction of PVDFwhich is not of very high mass. The total volume of CTA is not acritical parameter. It must be adjusted so as to fix the average molarmass of the polymer which is associated with the melt viscosity. Thevolume of water in which the dispersion of the monomers is made and theamounts of surfactant, initiator and CTA can be easily determined by aperson skilled in the art. The polymerization is carried out in astirred reactor and then the PVDF (in the form of solid particles) isseparated from the water by any means. These techniques are known per seand are described in Patents U.S. Pat. No. 4,025,709, U.S. Pat. No.4,569,978, U.S. Pat. No. 4,360,652, EP 626 396 and EP 655 468.

Advantageously, the aqueous emulsion is polymerized at a temperature of50 to 130° C.

Preferably, the polymerization is carried out at an absolute pressure of40 to 120 bar.

With regard to the surfactant: this denotes any product capable ofdispersing the monomers in water so as to facilitate theirpolymerization. U.S. Pat. No. 4,025,709, U.S. Pat. No. 4,569,978, U.S.Pat. No. 4,360,652, EP 626 396 and EP 655 468 describe processes forsynthesizing PVDF by putting the VF₂ into aqueous emulsion andpolymerizing it; many formulae of surfactants will be found in thesepatents.

As examples, mention may be made of those of general formula:ZC_(n)F_(2n)COOM in which Z is a fluorine or chlorine atom, n is aninteger ranging from 6 to 13 and M is a hydrogen or alkali metal atom oran ammonium group or an ammonium group having at least one lower alkylsubstituent.

Mention may also be made of lithium perfluoroalkanoates of formulaF₃C(CF₂)_(n-2)CO₂Li where n=7, 8, 9 or 10.

The amount of surfactant introduced at the start of or during thepolymerization may be between 0.01 and 5 parts per 100 parts of waterpresent in the initial charge of the reactor.

With regard to the water-soluble non-organic initiator capable ofpolmerizing the monomers, mention may essentially be made of inorganicperoxides, for example in the form of salts, such as sodium or potassiumpersulphate. The amount of initiator may be between 0.002 and 0.2 partsper 100 parts of monomers consumed in the reaction. Various coreactantswell known to those skilled in the art may also be added to theseinorganic peroxides to increase their rate of decomposition or to lowertheir temperature of use.

With regard to the organic initiator optionally employed to continue thereaction, mention may essentially be made of hydrocarbon peroxides suchas di-tert-butyl peroxide, dicumyl peroxide or benzoyl peroxide, dialkylpercarbonates, such as diethyl percarbonate or diisopropyl percarbonate,peracids or peresters, such as t-butyl perpivalate, t-amyl perpivalateor t-butyl peroxybenzoate.

With regard to the transfer agent, this also denotes any product whichmakes it possible to limit the molar mass of the polymer, while stillpropagating the polymerization reaction. As examples, mention may bemade of acetone, isopropanol, methyl acetate, ethyl acetate, diethylether, n-butyl acetate, diethyl malonate and diethyl carbonate, and ofvarious chlorofluorocarbon compounds. The amount of transfer agentessentially depends on its nature and on the desired average molar massof the polymer fraction obtained in its presence, which mass determinesthe average viscosity of the final product. Preferably, the transferagent used represents from 0.01 to 5 parts per 100 parts of monomersconsumed in the reaction.

EXAMPLES Example 1

18.6 l of deionized water and 50 g of a perfluorinated anionicsurfactant of the C₈F₁₇CO₂NH₄ type were introduced into a 30-litreautoclave.

The autoclave was closed and mechanically stirred intermittently, thenvacuum-degassed, filled with nitrogen at a pressure of up to 10 bar,vacuum-degassed again, filled with VF₂ at a pressure of up to 5 bar andfinally vacuum-degassed a last time. Next, the autoclave was heated to85° C., the mechanical stirring being at 150 rpm, and then filled withVF₂ at an absolute pressure of up to 85 bar.

A 50 ml dose of a 0.5% by weight aqueous solution of potassiumpersulphate was added in a single step in order to start the reaction.The pressure was maintained at 85 bar by the continuous introduction ofVF₂.

When 2.25 g of VF₂ had been consumed, 250 ml of ethyl acetate (EA) and70 ml of the same aqueous potassium persulphate solution were added.

When 9 kg of VF₂ had been consumed, 20 ml of aqueous persulphatesolution were incorporated in order to maintain a rate of conversionwithin the 1.5 to 3.5 kg/hour range.

When 9 kg of VF₂ had been consumed, the VF₂ feed was stopped and thereaction continued at a pressure of up to 42 bar before the temperaturewas lowered to 23° C. and the residual monomer removed from theautoclave.

This trial made it possible to produce 31.2 kg of a latex having asolids content of 39.5%, i.e. 12.3 kg of dry PVDF. The polymer fractionobtained before the transfer agent was introduced was 2.9 kg, i.e. 23.6%of the total of the product formed. The melt flow index measuredaccording to the ISO 1133 standard at 230° C. under a load of 5 kg was1.7 g/10 min.

Example 2

The procedure was as in Example 1, except for the additionalintroduction of 200 ml of EA when 9 kg of VF₂ had been consumed.

Example 3 VF2/HFP Copolymer

The procedure was as in Example 1, except for the introduction of 105 gof hexafluoropropylene (HFP) after the initial degassing and before thepressurization to 85 bar and the feed with a VF2/HFP mixture containing1% by weight of HFP up to 2.25 kg of mixture consumed. After introducingthe EA when the VF₂/HFP mixture had been added, the reaction wascontinued with just VF₂.

Example 4

The procedure was as in Example 1, except for the introduction of 480 mlof EA when 2.25 kg of VF₂ had been consumed.

Example 5

The procedure was as in Example 1, except for the introduction of 150 mlof EA when 1.2 kg of VF₂ had been consumed, and then 250 ml of EA when2.25 kg of VF₂ had been consumed.

Example 6 VF₂/CTFE Copolymer

The procedure was as in Example 1, except for the introduction of 50 gof chlorotrifluoroethylene (CTFE), after the initial degassing andbefore pressurization to 85 bar, and the feed with a VF₂/CTFE mixturecontaining 2% by weight of CTFE up to 2.25 kg of consumption of themixture. After introducing the EA when the VF₂/CTFE mixture had beenadded, the reaction was continued with just VF₂.

Example 7

The procedure was as in Example 1, except that the reaction was carriedout at an absolute pressure of 45 bar, this pressure being kept constantall the time by a continuous VF₂ feed. The amount of deionized waterinitially put into the autoclave was also reduced to 17.6 litres. Inaddition, instead of initially introducing a 50 ml dose of a 0.5%aqueous persulphate solution, an 80 ml dose of the same solution wasadded. Finally, when 1.7 kg of VF₂ had been consumed, a 60 ml volume ofEA was injected and the reaction continued by feeding the autoclave withan aqueous emulsion containing 2% by weight of n-propyl percarbonate(n-PP) at a rate of 300 ml/hour. The VF₂ feed was stopped when 7.1 kg ofVF₂ had been consumed and the addition of the n-PP emulsion maintainedat up to 10 bar, before the temperature was lowered to 23° C. and theresidual monomer removed from the autoclave.

The ABC Triblock

was a PMMA-PB-PS (PMMA-polybutadiene-polystyrene) triblock having thefollowing characteristics: the number-average molecular mass (M_(n)) ofthe PMMA blocks was 65 700 g/mol, the M_(n) of the PB blocks was 22 800and that of the PS blocks was 25 000. The ABC triblock was in fact amixture of 60% by weight of a pure ABC triblock and 40% of a pure BCdiblock, which was a synthesis intermediate; the BC diblock had a PCblock of 25 000 M_(n) and a PB block of 22 800 M_(n).

The PMMA block represented 58% as mass fraction of the total mass of thetriblock (i.e. of the sum of the pure triblock and the pure diblock),the PB block represented 20% as mass fraction of the total mass of thetriblock (i.e. of the sum of the pure triblock and the pure diblock) andthe PS block represented the remaining 22% (i.e. of the sum of the puretriblock and the pure diblock). It was prepared using the operatingmethod described in EP 524 054 or EP 749 987.

The properties of the PVDF are given in Tables 1-3. These tablesdescribe the mechanical and rheological properties of PVDF (orcopolymers with a very low HFP or CTFE content). The data must becompared with industrial products having a comparable melt flow indexlevel.

Firstly, with regard to the polymers suitable for being extruded(MFI≈2), that is to say Examples 1-3 and 6-7, it may be seen that thefracture energy at room temperature can be almost twice that ofKynar®1000 and four times that of Kynar® 740.

At −30° C., the effect is less pronounced, although an increase in thefracture energy of about 50% may be seen. On examining the tensileproperties, it may also be seen that there is a significant increase(+8%) in terms of the yield stress in Examples 2-3, which means that thematerial has a greater resistance to plastic deformation.

In the case of the polymers aimed at injection moulding or coextrusion(MFI≈15-20), the effect on the fracture energy is greater at −30C thanat room temperature, as demonstrated by the values obtained in the caseof Examples 4-5 compared with the commercial grades, Kynar® 720 and6000.

Based on the data obtained for the same products formulated with 10% ofABC triblock, Table 3 shows that the improvement in the impact behaviourresulting from the additive is decoupled at room temperature in thepresence of the FNTC.

Finally, two photographs obtained using an optical microscope underpolarized light from a microtome section on injection-moulded specimensallow the crystalline morphology of Kynar 740 to be compared with theproduct of Example 1. A reduction in the mean size of the spherulites isobserved in the case of the PVDF containing the FNTC.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius; and, unless otherwise indicated,all parts and percentages are by weight.

The entire disclosure of all applications, patents and publications,cited above and below, and of corresponding French Application No.0109453, filed Jul. 16, 2001 is hereby incorporated by reference. TABLE1 Charpy Tensile properties MFI impact Yield Yield Tensile Elong. atRheology 230° C./5 kg −30° C. 23° C. stress elong. strength break (Pa ·s) Examples g/10 min (kJ/m²) (kJ/m²) (MPa) (%) (MPa) (%) 10 s⁻¹ 100 s⁻¹1000 s⁻¹ Observations Kynar ® 2 7.2 22.8 48.0 10.2 23.4 118 6625 1791409 1000 PVDF homopolymer Kynar ® 740 2 6.6 12.0 49.9 9.4 27.5 192 67411698 394 PVDF homopolymer Ex 1 1.7 10.1 40.3 45.9 11.0 29.9 183 90922313 478 ≃24% of non-transferred chains (chains of very high mass) Ex 22.4 9.6 35.6 53.3 10.1 35.8 134 8077 2110 455 +200 ml of EA after 9 kgof VF₂ had been consumed Ex 3 2 8.9 33.8 54.0 11.3 34.6 153 7996 2129456 +HFP Ex 6 1.8 9.5 41.8 43.8 10.6 35 255 13178 3388 714 +CTFE Ex 71.6 7.1 16.4 50.6 11 35.2 222 6882 1662 351 +n-PP after 1.7 kg of VF₂had been consumed

TABLE 2 Charpy Tensile properties MFI impact Yield Yield Tensile Elong.At Rheology 230° C./5 kg −30° C. 23° C. stress elong. strength break (Pa· s) Examples g/10 min (kJ/m²) (kJ/m²) (MPa) (%) (MPa) (%) 10 s⁻¹ 100s⁻¹ 1000 s⁻¹ Observations Kynar ® 19 3.9 7.3 53.3 10.6 25.6 103 2422 908248 720 PVDF homopolymer Kynar ® 18 3.9 8.5 51.1 9.9 31.4 40 2540 916272 6000 PVDF homopolymer Ex 4 20.8 6.7 10.7 52.4 9.7 52.4 27 2369 891260 ≃24% of non-transferred chains EX 5 16 9.1 11.9 46.7 11.8 32.0 672839 1023 291 ≃13% of non-transferred chains + 250 ml of EA after 2.25kg of VF₂ had been consumed

TABLE 3 Charpy Tensile properties MFI impact Yield Yield Tensile Elong.Rheology 230° C./5 kg −30° C. 23° C. stress elong. strength at break (Pa· s) Examples g/10 min (kJ/m²) (kJ/m²) (MPa) (%) (MPa) (%) 10 s⁻¹ 100s⁻¹ 1000 s⁻¹ Observations Kynar ® 720 + 9.2 15.8 41.2 8.2 23.2 24 10%SBM Ex 4 + 10% 9.0 41.0 43.8 7.1 20.5 39 SBM Ex 5 + 10% 10.2 38.6 45.38.2 29.5 95 SBM

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1-10. (canceled)
 11. A blend of a PVDF (polyvinylidene fluoride)homopolymer or copolymer of a comonomer containing a vinyl group capableof being opened by the action of free radicals in order to bepolymerized and which contains, directly attached to this vinyl group,at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxy group,said PVDF for comprising: a fraction of non-transferred chains of whichare insoluble in DMF, DMSO and NMP, said fraction having a dynamicviscosity of greater than 50 kpoise at 230° C. and at a shear rate of100 s⁻¹, and a spherulites size of 0.5 to 4 μm, with an ABC triblockcopolymer, blocks A, B and C being linked together in this order, eachblock being either a homopolymer or a copolymer obtained from two ormore monomers, the A block being linked to the B block and the B blockto the C block by means of a covalent bond or of an intermediatemolecule linked to one of these blocks via a covalent bond and to theother block via another covalent bond, and such that: the A block iscompatible with PVDF, the B block is incompatible with PVDF and isincompatible with the A block, the C block is incompatible with PVDF,the A block and the B block
 12. A blend according to claim 11, whereinthe PVDF is a vinylidene fluoride (VF₂) homopolymer or copolymer of acomonomer which is chlorotrifluoroethylene (CTFE), hexafluoropropylene(HFP), trifluoroethylene (VF₃) or tetrafluoroethylene (TFE).
 13. A blendaccording to claim 11, in which the proportion of non-transferred chainsin the PVDF is 2 to 30% by weight.
 14. A blend according to claim 13, inwhich the proportion of non-transferred chains in the PVDF is between 5and 30% by weight.
 15. A blend according to claim 14, in which theproportion of non-transferred chains in the PVDF is between 15 and 25%by weight.
 16. A blend according to claim 11, in which the size of thespherulites in the PVDF is 0.8 to 2 μm.
 17. Blends according to claim11, in which the ABC triblock is poly(methylMethacrylate-b-butadiene-b-styrene).
 18. Parts made of the materialaccording to claim
 11. 19. A process for synthesizing a blend accordingto claim 11, in which the PVDF is prepared by: a dispersion of VF₂(vinylidene fluoride) and of one or more optional comonomers in water ismade, optionally with the aid of a surfactant, said dispersion beinginitially brought into contact with a water-soluble non-organicinitiator capable of causing the polymerization of the monomers; part ofthe PVDF having been formed in the presence of the water-solublenon-organic initiator, the following are added: either (i) a chaintransfer agent capable of propagating the polymerization, the saidpolymerization then being initiated by a water-soluble non-organicinitiator or by an organic initiator, or (ii) an organic initiator, alsocapable of accomplishing chain transfer, and optionally a water-solublenon-organic initiator.
 20. A blend according to claim 12, in which thePVDF contains at least 50% by weight of VF₂.
 21. A blend according toclaim 11, in which the proportion of non-transferred chains in the PVDFis up to 50% by weight.
 22. A blend of a (polyvinylidene fluoride)homopolymer or copolymer of a comonomer containing a vinyl group capableof being opened by the action of free radicals in order to bepolymerized and which contains, directly attached to this vinyl group,at least one fluorine atom, a fluoroalkyl group or a fluoroalkoxy group,said PVDF comprising a fraction of non-transferred chains of which areinsoluble in DMF, DMSO and NMP, said PVDF being produced by a processcomprising polymerization in the absence of a transfer agent and with aninhibitor which does not induce a transfer reaction, whereby PVDF isformed, and subsequently continuing polymerization in the present of atransfer agent, with an ABC triblock copolymer, blocks A, B and C beinglinked together in this order, each block being either a homopolymer ora copolymer obtained from two or more monomers, the A block being linkedto the B block and the B block to the C block by means of a covalentbond or of an intermediate molecule linked to one of these blocks via acovalent bond and to the other block via another covalent bond, and suchthat: the A block is compatible with PVDF, the B block is incompatiblewith PVDF and is incompatible with the A block, the C block isincompatible with PVDF, the A block and the B block.
 23. A blendaccording to claim 22, wherein the fraction of non-transferred chains inthe PVDF has a dynamic viscosity of greater than 50 kpoise at 230° C.and at a shear rate of 100 s⁻¹, and a spherulites size of 0.5 to 4 μm.24. A blend according to claim 22, in which the proportion ofnon-transferred chains in the PVDF is up to 50% by weight.